The objective of this research project is to characterize the plasma lipoprotein source of free cholesterol which is utilized by the liver for bile acid synthesis and biliary cholesterol secretion. The sources and fate of the esterified and free cholesterol in individual lipoprotein classes (VLDL, IDL, LDL and HDL) as well as RBC is also being investigated in man in vivo. These studies are an extension of our ongoing multicompartmental analysis of cholesterol metabolism in man which has shown that plasma free colesterol is the major source of the biliary steroids. Multicompartmental analysis of the data obtained during the past year in patients given HDL free 3H cholesterol and LDL or VLDL-free 14C cholesterol is underway. The preliminary data show that most of the cholesterol in bile is derived from HDL rather than Apo B containing lipoproteins. We are now using the swine perfused liver preparation to determine the mechanism of the preferential uptake of cholesterol from HDL. In a study recently completed, we found that bile acid synthesis and biliary cholesterol secretion declined in the absence of lipoproteins in the perfusate, and that the perfused pig liver, like man, preferentially utilizes the lipoprotein free cholesterol for secretion. In present and future studies, livers will be perfused with either HDL, LDL, VLDL, or a combination of two lipoproteins. The specific versus non-specific uptake of free cholesterol from lipoproteins will be studied by varying the HDL and LDL perfusate concentrations, and the order of administration to the liver. We have undertaken an extensive, complex multicompartmental analysis of plasma esterified (and free) cholesterol metabolism in man. The initial analysis has shown that 80% of the esterified cholesterol in HDL is derived from LDL ester, and 20% is newly synthesized from HDL-free cholesterol (LCAT).